Hormonal and metabolite effects on plasma free fatty acids in the northern pike, Esox lucius L

In vivoregulation of rainbow trout lipolysis by catecholamines

Lipolysis provides fatty acids that support key life processes by functioning as membrane components, oxidative fuels and metabolic signals. It is commonly measured as the rate of appearance of glycerol(Ra glycerol). Its in vivo regulation by catecholamines has been thoroughly investigated in mammals, but little information is available for ectotherms. Therefore, the goals of this study were, first, to characterize the effects of the catecholamines norepinephrine(NE) and epinephrine (Epi) on the lipolytic rate of intact rainbow trout(Oncorhynchus mykiss) and, second, to determine whether the plasma glycerol concentration is a reliable index of Ra glycerol. Our results show that baseline Ra glycerol (4.6±0.4μmol kg–1 min–1) is inhibited by NE(–56%), instead of being stimulated, as in mammals, whereas Epi has the same activating effect in both groups of vertebrates (+167%). NE-induced inhibition of fish lipolysis might play a particularly important role during aquatic hypoxia, when survival often depends on regulated metabolic depression. The plasma glycerol concentration is a poor predictor of Ra glycerol, and it should not be used as an index of lipolysis. Trout maintain a particularly high baseline lipolytic rate because only 13% of the fatty acids provided are sufficient to support total energy expenditure, whereas the remaining fatty acids must undergo reesterification(87%).

The adrenergic control of hepatic glucose and FFA metabolism in rainbow trout (Oncorhynchus mykiss): increased sensitivity to adrenergic stimulation with fasting

The adrenergic control of glucose and FFA release was studied in hepatocytes of rainbow trout (Oncorhynchus mykiss), which were either normally fed or fasted for 3 weeks. Isolated hepatocytes were incubated with adrenaline, noradrenaline, or isoprenaline (nonselective beta-agonist). Identification of the hepatic beta-adrenoceptor was combined with quantification of the difference in its affinity for adrenaline and noradrenaline. To identify the beta-adrenoceptor subtype, isoprenaline incubations were combined with atenolol (selective beta(1)-antagonist) or ICI 118,551 (selective beta2-antagonist). Stimulation of the beta-adrenoceptor resulted in mobilisation of glucose, which was inhibited by ICI 118,551 thus pointing to a beta2-subtype. The affinity of the beta2-adrenoceptor for isoprenaline and adrenaline (beta2-values of 8.3 and 7.9) was clearly higher than for noradrenaline (beta2-value of 6.5). This indicates that at physiological concentrations beta2-adrenoceptors in trout are mainly stimulated by adrenaline and not by noradrenaline. A significant effect of beta-adrenoceptor stimulation on the FFA release was also found, although only at high concentrations (i.e., 10(-6) and 10(-5)M). Again the beta2-adrenoceptor appeared to mediate the stimulation of hepatic FFA release. Upon fasting, both the basal glucose and FFA release were strongly decreased. The ratio between glucose and FFA release decreased from 15.4 to 4.3 upon fasting and at this ratio the energy output for both metabolites became equal. The mobilisation of FFA upon adrenergic stimulation was relatively conserved, namely -35% upon fasting, as opposed to -89% in mobilisation of glucose. This indicates that upon fasting FFA gain importance in hepatic metabolism. The hepatic sensitivity to adrenergic stimulation is enhanced upon fasting, as indicated by an increased beta2-value from 8.3 to 8.9.

Beta-adrenergic control of plasma glucose and free fatty acid levels in the air-breathing African catfishClarias gariepinusBurchell 1822

In several water-breathing fish species, β-adrenergic receptor stimulation by noradrenaline leads to a decrease in plasma free fatty acid(FFA) levels, as opposed to an increase in air-breathing mammals. We hypothesised that this change in adrenergic control is related to the mode of breathing. Therefore, cannulated air-breathing African catfish were infused for 90 min with noradrenaline or with the nonselective β-agonist,isoprenaline. To identify the receptor type involved, a bolus of either a selective β1-antagonist (atenolol) or a selectiveβ 2-antagonist (ICI 118,551) was injected 15 min prior to the isoprenaline infusion. Both noradrenaline and isoprenaline led to an expected rise in glucose concentration. Isoprenaline combined with both theβ 1- and β2-antagonist led to higher glucose concentrations than isoprenaline alone. This could indicate the presence of a stimulatory β-adrenoceptor different from β1 andβ 2-adrenoceptors; these two receptors thus seemed to mediate a reduction in plasma glucose concentration. Both noradrenaline and isoprenaline led to a significant decrease in FFA concentration. Whereas theβ 1-antagonist had no effect, the β2-antagonist reduced the decrease in FFA concentration, indicating the involvement ofβ 2-adrenoceptors. It is concluded that the air-breathing African catfish reflects water-breathing fish in the adrenergic control of plasma FFA and glucose levels.

Free fatty acid metabolism in the air-breathing African catfish (Clarias gariepinus) during asphyxia

In several waterbreathing fish species, hypoxia induces a decrease in plasma free fatty acid (FFA) levels as opposed to an increase in air-breathing mammals. We hypothesised that this change is coupled to the mode of breathing. Therefore, we followed the metabolic response of cannulated air-breathing African catfish to an 8-h asphyxia period. The hematocrit and hemoglobin increased significantly upon asphyxia. However, no change was observed in the mean cellular hemoglobin concentration, indicating that more erythrocytes were brought into circulation. A continuous increase in plasma lactate concentration during asphyxia showed permanent activation of anaerobic glycolysis, pointing to a persistent oxygen shortage. Plasma glucose levels did not change, but FFA levels decreased significantly upon asphyxia with a concomitant increase in plasma noradrenaline levels. Thus, these results suggest that in the air-breathing African catfish noradrenaline mediated a decrease in plasma FFA levels similar to that in waterbreathing fish species.

Insulin stimulates hepatic lipogenesis in rainbow trout, Oncorhynchus mykiss

The effects of the pancreatic hormones, insulin and glucagon, on rates of lipid biosynthesis in liver removed from rainbow trout, Oncorhynchus mykiss, were evaluated in vitro. Livers were removed from animals fasted for 30-36h, cut into ca. 1 mm(3) pieces, and incubated in the presence of various concentrations of salmon insulin (sINS), bovine insulin (bINS), or a combination of BINS and bovine/porcine glucagon (GLU). Lipid synthesis was evaluated by total lipid concentration, (3)H2O incorporation into total lipid, and by fatty acid synthetase activity. Both mammalian and sINS tended to increase tissue total lipid concentration in hepatic tissue incubated for 5h. Insulin also stimulated (3)H2O incorporation into total lipid in a dose-dependent manner. Bovine INS (2 × 10(-6) M) stimulated de novo synthesis nearly 6-fold over control rates; sINS (2 × 10(-6) M) stimulated label incorporation more than 7-fold over control rates. Glucagon inhibited INS-stimulated (3)H2O incorporation; whereas, GLU alone had no effect on lipid synthesis in liver pieces incubated 5h. Lipid class analysis indicated that bINS significantly stimulated (3)H2O incorporation into phospholipids, fatty acids, and triacylglycerols. The greatest accumulation of label was in the triacylglycerol fraction, where incorporation was stimulated 17-fold over control levels. Hepatic enzymatic analysis indicated that bINS also significantly stimulated lipogenic enzyme activity 9-fold above control levels. These results indicate that INS is an important regulator of lipid synthesis in the liver of trout.

Changes in plasma glucagon, insulin and tissue metabolites associated with prolonged fasting in brown trout (Salmo trutta fario) during two different seasons of the year

1. Pyrenean brown trout juveniles (Salmo trutta fario) were fasted for 50 days in late winter (experiment 1) and summer (experiment 2). Plasma insulin, glucagon and glucose and some metabolites in plasma and in tissues were analysed. 2. Glucagon increased significantly on the 3rd day of fasting in the winter experiment (controls 653.7 +/- 92.4 pg/ml, fasted 912.7 +/- 135.2 pg/ml), and the same tendency was observed on the 5th day in the summer experiment (controls 430.5 +/- 56.2 pg/ml, fasted 555.5 +/- 95.3 pg/ml). During this initial period of fasting, plasma glucose was maintained in both experiments (75.5 +/- 4.7-67.6 +/- 4.1 mg/100 ml), but from day 8, glucose and glucagon decreased simultaneously. 3. Insulin decreased from the beginning of fasting, reaching lowest values after 50 days of fasting (winter experiment: controls 6.4 +/- 0.3 ng/ml, fasted 1.6 +/- 0.1 ng/ml; summer experiment: controls 4.8 +/- 0.1 ng/ml, fasted 1.2 +/- 0.2 ng/ml). Glucagon/insulin molar ratio (G/I) increased after 3 days in the winter experiment (controls 0.21 +/- 0.02, fasted 0.39 +/- 0.05), while in the summer experiment, the ratio rose from day 5 and reached a peak at day 30 (controls 0.16 +/- 0.02, fasted 0.48 +/- 0.07). 4. Muscle proteins were significantly mobilized after 50 days of fasting. Visceral index decreased significantly after day 15 while liver glycogen was already significantly lower at day 8. However, in the summer experiment, a transitory increase of liver glycogen was observed at day 30, coinciding with the peak of G/I.

Effects of nutritional state, insulin, and glucagon on lipid mobilization in rainbow trout, Oncorhynchus mykiss

The effects of nutritional state, insulin, and glucagon on lipid mobilization were determined in rainbow trout, Oncorhynchus mykiss. In nutritional state experiments, fish were either fed continuously (except 24 to 36 hr prior to experimentation) with commercial trout chow or fasted for 4 weeks. Lipase activity in liver tissue isolated from fasted fish and cultured for 5 hr was greater than that in tissue isolated from fed fish and cultured. The presence of glucose (5.55 mM) in the incubation medium accentuates lipolytic activity in both liver and adipose tissue. Hormone response was assessed both in vivo and in vitro. Salmon insulin was injected into anesthetized fish (fed continuously except 24 hr prior to injections) in 10 microliters of saline/g body weight; final hormone dose was 100 ng/g body weight. Tissue and plasma were sampled 1 and 3 hr after injection. Insulin resulted in depressed plasma FA concentration and reduced hepatic triacylglycerol lipase activity. In vitro effects of hormones were evaluated by incubating liver and adipose tissue pieces in Hanks-MEM. Glucagon (bovine/porcine) directly stimulated lipid breakdown in both liver and adipose tissue. These actions were manifested by enhanced FA and glycerol released into the culture medium and by elevated triacylglycerol lipase activity. Insulin (bovine) generally appeared antilipolytic as this agent inhibited glucagon-stimulated lipase activity and glucagon-stimulated FA release. Furthermore, insulin (in the presence of glucose) reduced net lipolysis, as indicated by glycerol release, compared to control cultures. These results indicate that nutritional state and glucose are important modulators of lipid mobilization and that glucagon and insulin act directly on lipid storage sites to coordinate lipolysis in rainbow trout.

Interactive effects of high stocking density and triiodothyronine-administration on aspects of the in vivo intermediary metabolism and in vitro hepatic response to catecholamine and pancreatic hormone stimulation in brook charr, Salvelinus fontinalis

Brook charr (Salvelinus fontinalis) were maintained at one of two stocking densities (SD) (30 or 120 kg/m3) and fed either a control or a T3-supplemented (20 mg/kg) diet for 30 days in order to investigate possible interactive effects of SD and T3-administration on growth, feeding rate, food conversion efficiency, and hepatic and dark muscle enzyme activity. In addition, liver slices were incubated in vitro for 6 h with epinephrine, norepinephrine, isoproterenol, propranolol, insulin, glucagon, or somatostatin to evaluate possible SD-T3 interactive effects on hepatic responses to hormonal stimulation. Maintaining the fish at high SD appeared to increase the clearance rate of T3 from the T3-supplemented group. There was no clear evidence of SD-T3 interactive effects on growth rate, feeding rate, or food conversion efficiency, although T3-administration decreased food conversion efficiency, and high SD decreased growth and feeding rates. Of the hepatic enzymes studied, HOAD, malic enzyme, G6PDH, CS, PFK, HK, and GDH activities all showed changes suggestive of interactive SD-T3 effects. Although hepatic FBPase was stimulated by both high SD and T3-administration, there was no evidence of interactive SD-T3 effects. Dark muscle HOAD, CS, and PFK also showed SD-T3-related responses; dark muscle malic enzyme, G6PDH, HK, and GDH were unaffected by either altered SD or T3-administration. Prior treatment of the fish with T3 and high SD had significant effects on free fatty acid (ffa) release to the medium and on hepatic lipid content, but had no effect on the responses to the various endocrine agents used. Glucose release from liver slices of fish stocked at high density (both T3-supplemented and controls) was higher than that of the fish stocked at low density; with the exception of insulin and glucagon, glucose release was similar in all pre-treatment groups. The insulin- and glucagon-stimulated changes in glucose release seen in the fish fed non-supplemented diets were not found in the two groups of fish fed the T3-supplemented diets. High SD and/or T3-administration induced significant lowering of hepatic glycogen content, but there was no effect of pre-treatment on the response to any of the endocrine agents used. The data show a marked effect of SD on energy partitioning processes in brook charr and the animal's ability to respond to T3-stimulation, but provided no evidence of such effects on the liver response to the various agents used.

Metabolic actions of glucagon-family hormones in liver

This review addresses direct and indirect metabolic actions of hormones co-encoded in the preproglucagon gene of fishes. Emphasis is placed on a critical analysis of the effects of glucagon and glucagon-like peptide (GLP) and the current knowledge of the respective modes of action is reviewed. In mammals GLPs exert no direct metabolic actions. In fish liver, GLP and glucagon act on similar targets of intermediary metabolism by enhancing flux through glycogenolysis, lipolysis and gluconeogenesis. Increases in substrate oxidation are not uniform. Hormonal activation of glycogen phosphorylase and triglyceride lipase and inhibition of pyruvate kinase are implicated in these actions. Hormone-dependent hyperglycemia, depletion of hepatic glycogen and increases in free fatty acids are noticeablein vivo. Glucagon also activates hepatic amino acid uptake and ammonia excretion.Glucagon actions are accompanied by large increases in hepatic cAMP and increased phosphorylation of pyruvate kinase. Metabolic effects measured after GLP administration are associated with minor, if any, increases in cAMP and effects on pyruvate kinase are variable. We hypothesize that different hepatic receptors with differing modes of intracellular message transduction are involved in glucagon and GLP actions while targetting identical metabolic routes. Responses of different species of fish cover a wide spectrum, and variation of response with the circannual cycle of experimental animals makes comparisons of results, even within one species, difficult.

Lipid dynamics in fish: aspects of absorption, transportation, deposition and mobilization

1. Aspects of lipid metabolism, including absorption and depositional processes, appear quite different in fish as compared to homeothermic vertebrates. 2. Dietary lipids in fish are absorbed as fatty acids and as triacylglycerols aggregated into chylomicra particles. 3. Interorgan transport of lipids, like that of mammals, consists of an exogenous (dietary) loop and an endogenous loop. 4. Fish store lipids among several depot organs, including mesenteric membranes, liver and muscle. 5. Several fast-acting and slow-acting agents modulate depot lipid mobilization. 6. Mobilized lipids may be transported in the serum as free fatty acids bound to specific carrier proteins.

Immunocytochemical localization of hormone-producing cells within the pancreatic islets of the rainbow trout (Salmo gairdneri)

A histological study of the pancreatic islets in rainbow trout, Salmo gairdneri, was undertaken in which polypeptide hormones-producing cells were localized, using immunocytochemical staining techniques. Four different cell-types were identified in this manner. These were the insulin, somatostatin, pancreatic polypeptide and glucagon/gastric inhibitory polypeptide (GIP) cells. The glucagon/GIP cell was designated thus as antisera to both hormones cross-reacted with a common population of cells. A fifth cell-type, commonly referred to as a clear cell, was also identified although its secretory product is as yet undetermined. These functional cell types were compared to the standard tinctorial properties of pancreatic endocrine cells. The relationships of the various cell types with each other was also observed.